The aims of this Core are to: 1. Conduct cross-project and secondary analyses to assist in the identification and characterization of high-risk subgroups. 2. Support the current work of funded projects by assisting them in determining the generalizability of project findings over time (using historical data) and geographically (both across different part of New York City and by comparison with data from other localities (using data from other funded projects as well as historical data sets). 3. Develop methodological innovations that will assist investigators on HIV and drugs in the field as a whole and in current and future Center projects. Specific foci of this work will involve developing methods to study high-risk networks and to compare data from different projects.